PROJECT SUMMARY/ABSTRACT Glucocorticoid-induced osteoporosis is the most common drug-induced form of osteoporosis. Fractures may occur in 30 to 50% of patients receiving chronic glucocorticoid therapy but bone density measurements underestimate the risk of fracture in this disorder and account for, at least in part, the under- recognition and under-treatment of the disease. Vertebral and rib fractures are of particular importance to the VA patient care mission because in addition to causing pain and reducing physical activity, they can markedly decrease respiratory function in patients with pulmonary disease by causing splinting of the chest wall, thereby reducing the capacity to breathe. Glucocorticoid therapy causes a decline in bone strength that surpasses the decline in bone density, but little is known of the mechanism behind this phenomenon. Although it is widely appreciated that bone is composed of cells, mineral and collagen, it is seldom realized that water is another major component accounting for more than one fourth of the wet weight of bone. Fracture resistance of hard tissues is defective without water and water confers to bone much of its unique strength and resilience. The long-term objective of this proposal is to determine whether drug-induced deterioration of bone water and vascularity may account for the disproportionately greater decline in bone strength than in bone mass typical of glucocorticoid- induced osteoporosis and whether the bone strength and vascularity may be compromised through direct actions of glucocorticoids on bone cells. Studies aimed at the cellular and molecular mechanisms of the pathogenesis of the loss of bone strength in glucocorticoid-induced osteoporosis and investigation of the reasons for the limited response to current therapy would increase motivation to protect the skeleton as early as possible, before there are decreases in bone density. To achieve this goal, the direct effects of glucocorticoid excess on the expression of vascular proteins from bone cells will be investigated. Next, determination of the contribution of bone water and vascularity to the loss of bone strength in an animal model of glucocorticoid-induced osteoporosis and the additional impact of various treatment strategies to prevent glucocorticoid-induced osteoporosis will be examined. The clinical significance of these changes in bone vascular factors will be elucidated in humans using archival bone biopsy specimens obtained from patients with postmenopausal osteoporosis or glucocorticoid-induced osteoporosis. The studies proposed in this application are timely and by capitalizing on modern concepts and innovative methodology offer the opportunity for new insights that are sorely needed for the prevention and treatment of glucocorticoid-induced bone disease and are, therefore, immediately relevant and vital to the VA health care mission.